1. Field of the Invention
The invention relates to rotary drill bits for use in milling a casing window, and for use in drilling subterranean earthen materials.
2. Description of Related Art
After a wellbore has been drilled into subterranean earthen material, a casing is cemented into place to provide protection against pollution of water aquifers. With the advent of improved directional drilling techniques, existing wellbores are being used as starting points from which new, lateral boreholes are drilled. In order to initiate the drilling of a lateral borehole, an opening or window must be cut or milled into the casing. A curved drilling guide or “whipstock” is set in the casing, and a special milling tool is lowered into the casing. The whipstock directs the milling tool against the casing wall, and the rotation of the milling tools creates the casing window. Once the casing window has been created in harder formations, the milling tool must be removed from the casing and a different drill bit used to drill the lateral borehole in the subterranean earthen material.
The use of a milling tool to create the casing window and the use of a different drill bit to drill the lateral borehole causes significant waste of time waiting for the drill string to be removed and then reentered into the casing. With offshore drilling rig day rates being so expensive, there is strong economic incentive to reduce the number of “trips” into and out of the wellbore. Therefore, there is a need for a drill bit that can be used for both milling of the casing window and for drilling the lateral wellbore, without the need for a drill string trip out of and back into the wellbore.